In general, a seatbelt installed in a vehicle functions to restrain the body of a passenger in case of an accident so as to reduce a shock delivered to the passenger and to prevent the passenger from colliding with the interior of the vehicle due to inertial force. Most automobiles employ a three-point type seatbelt that is composed of a shoulder belt for securing the upper body of a passenger and a lap belt for securing the waist of a passenger.
As well known to the public, safety and reliability of seatbelts has been proved through repeated collision tests and the seatbelt, therefore, is an inevitable safety system in automobiles for protecting passengers in case of an accident. Recently developed seatbelts include webbing control means that adjust the restraining point of a seatbelt in accordance with the size of the passenger, or a retractor that reduces the restriction of movement of the passenger in a normal state while sufficiently protecting the passenger in case of an accident.
The retractor, one element of a seatbelt assembly, is a device for retracting the webbing. It typically includes means that prevent the webbing from being drawn out in an abnormal state, such as in a collision. Because the performance of a seatbelt mainly depends on the retractor, various types of retractors have been developed for improving safety and reliability of a seatbelt. Such retractors are disclosed, for example, in U.S. Pat. Nos. 6,254,191, 5,232,177, 4,436,255.
Prior retractors, however, can be disadvantageous in that the webbing can be excessively wound up due to an unregulated restoring force of the spiral spring installed on the ratchet wheel. A mechanical shock is generated by sudden expansion of the spiral spring, which results in locking of a vehicle sensor, namely, unintentional engagement of a ratchet wheel and a latch. Because the unintentional engagement of the ratchet wheel and the latch occurs when a webbing is fully wound up, it can be mechanically impossible to release the engagement without taking apart the elements.